Digging and cleaning up Pocket collection, found several posts which propose another idea of how to implement Responsive Images. One of them is proposed by Morten Rand-Hendriksen here (dated June 2015):

Those are nice ideas for responsive images. But since Responsive Images already at WordPress core, hence these are more for house cleaning. Will not look (or use them at the moment) as the focus is for getting better understanding of the official one.

Moving on, get this post by Smashing Magazine. Usually, articles from SM give a good explanation about new things. But unfortunately, this is just another introductory about the new feature. Good, but need to dig deeper (for how to really implement it in a theme).